Telescope Making

Pitch Tester

In order to achieve the best results during polishing one needs to test the hardness of the pitch. Traditionaly the thumnail test have been used but it does not yield repeatable results. Repeatable results can only be achieved with a pitch tester. In order to get comparable results, the tester below is based on the tester described by Texereau. The tester consists of a sharp point which is pressed into the pitch specimen by a 1 kg weight. The point has an angle of 15 degrees with a flat point of 1 mm.

Here is a few photos of the pitch tester that I have made. Click on the thumbnail to see a bigger picture. The frame is made from 25 mm rectangular pipe which is welded together. The weight is a 50 mm rectangular block of steel which weighs about 1 kg. In total the weight, axle and point weighs 996 grams. The point of the tester is adjustable to accommodate pitch of different thicknesses.

Tester seen from the front. Tester seen from the side. Upper part of the tester. Lower part of the tester. The point can be adjusted with the set screw.
     
Piece of pitch under test.      

 

The hardness is determined by measuring the total fall in 5 minutes. One might think that one can only measure the fall rate over a shorter period and then calculate the fall over 5 minutes but unfortunately the fall rate is not linear with time. Initially the fall rate is high and then decreases over the 5 minutes. In order to get data of the fall rate, I tested 3 pieces of pitch over 5 minutes by making a video of the fall during the test. One of the pieces is a pitch lap which is a few years old. The pitch was obtained from Willman-Bell and is about 6 mm thick. The other 2 pieces is Bitumen made from tar. The pieces is 7 and 9 mm thick and came from the same piece of pitch. Below is a graph of the results:

It can be seen from the graph that initially the fall rate is high and then decreases and at the end of the 5 minutes becomes very slow. The two Bitumen pitch pieces gave nearly the same results which means that thickness does not play a role as long as the thickness is at least a few mm more than the total fall in 5 minutes. The Bitumen fall rate is initially higher than that of the Willman-Bell pitch. It is also obvious that the Bitumen is harder than the Willman-Bell pitch, which is in contradiction with the thumbnail test. The thumbnail test shows that the Bitumen pitch is soft as it is easily dented by the nail whereas the Willman-Bell pitch is not dented as easily. It shows the subjectivity of the thumbnail test and is therefore not a good test to use. If it is used it should be used only on the same type of pitch as it seems that different kinds of pitch behaves differently in the test.

How hard should te pitch be? Texereau's limit for soft pitch is 6.10 mm (0.24”) per 5 minutes for f/6 mirrors and a hard limit of 0.51 mm (0.02”) per 5 minutes for f/12 mirrors. If we assume a linear relationship we get the following equation:

vmax = -0.932*f + 11.70

where vmax is the softest that the pitch can be and f is the f ratio. Temperature also plays a role in the hardness of pitch. The warmer it is, the softer the pitch becomes.

f-ratio Fall (mm per 5 minutes) Fall (inch per 5 minutes)
f/4 7.97 0.31
f/5 7.04 0.28
f/6 6.10 0.24
f/7 5.18 0.20
f/8 4.24 0.17
f/9 3.31 0.13
f/10 2.38 0.09
f/11 1.45 0.06
f/12 0.76 0.02

 

Mirrors with lower f-ratios needs softer pitch in order to follow the steeper curve.

See also Eric Shrader's hardness comparisons of different pitch types.